Radiographic assessment of alignment following TKA: outline of a standardized protocol and assessment of a newly devised trigonometric method of analysis

Background: An important determinant of long-term outcomes following total knee arthrolplasty (TKA) is post-operative alignment as measured on radiographs. Thus far, radiographs have been measured using the goniometer method (GM) and no standard protocol has been followed. The aim of this prospective study was to: (i) outline a protocol for radiographic measurement following TKA; and (ii) compare the accuracy of the traditional GM with a new trigonometric method (TM) of radiographic analysis. Methods: A protocol for the measurement of alignment on radiographs following TKA was outlined in detail with step-by-step instructions. A new TM of angle measurement was also delineated. Alignment was measured on 51 post-operative TKA radiographs. A single angle was chosen and measured by two observers using both the GM and TM. Results: The TM had a precision of 1.06 degrees compared with 1.5 degrees using the GM. The standard deviation of the TM was significantly smaller than the GM (P = 0.033). The intra-class correlation coefficient of the TM was 0.94 versus 0.90 for the GM. Conclusion: The study detailed a protocol for the measurement of axial alignment of the limbs and components following TKA, and provided evidence that a newer TM of angle measurement was superior in terms of precision and intra-rater reliability in comparison with the traditional method.